Zone refining process



Jan. 14, 1964 5,5. CHANDRASEKHAR 3,117,859

ZONE REFINING PROCESS Filed Dec. 30, 1957 5 V/ %r JlA f? 44k L 6 WITNESSES INVENTOR Be llur S. Chundrosekhgr BY 9%. @M.

ilnited htates 3,117,859 ZGNE REFINING PROCESS Beilur S. Chandrasekhar, Pittsburgh, Pa, assignor to Westinghouse Eiectric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Dec. 30, 1957, Ser. No. 706,036 5 Claims. (Cl. 7510) This invention relates to a process for zone refining metals and semi-conductors employing improved floating zone techniques.

The zone refining of metals, metal alloys, semi-conductors and like materials, is Widely practiced for the purpose of producing materials of high purity. The zone refining process may be applied to some metals while they are disposed in a crucible or boat providing that the material eing zone refined is not reactive with the material forming the crucible or boat. However, high melting point materials, for example silicon, melt at such an elevated temperature and have such reactivity when melted that no known crucible material can be employed with completely satisfactory results.

it has been proposed to zone refine a bar or rod of a material, for example, such as silicon, while it is supported vertically without the use of any crucible or other confining receptacle. This procedure is commonly known as a floating zone process. In such vertical zone floating zone process the bar is surrounded by a heating member such, for example, as an electrically heated graphite ring or an induction coil whereby a relatively narrow zone of it is molten. lroviding the diameter of the bar is not excessive and the length of the molten zone is relatively thin, surface tension forces will maintain the molten zone in place. By relative movement of the heating device to the silicon bar there is obtained a relative movement of the molten zone such that refining of the entire bar of silicon may be secured. When the diameter of the bar of ilicon is greater than approximately 12 millimeters the length of the molten zone that may be practically produced becomes so great that the surface tension force becomes inadequate to support the molten globule in place and it will break and run out. This ruins the bar and, of course, terminates the zone melting process.

The object of the present invention is to provide a process for passing an electrical current longitudinally along a length of a bar of material concurrently with the heating of a zone of the bar so that a molten zone is produced and the electrical current gives rise to an electromagnetic pinch efiect which assists the molten globule of material to remain in place.

A further object of the invention is to assist in maintaining a molten zone of material in place in a bar material being zone refined by passing a strong electrical current longitudinally along the bar whereby to produce an electromagnetic pinch effect.

Other objects of the invention will, in part be obvious and in part will be described hereinafter. Reference should be had to the following drawing in which the single figure is a vertical cross section through apparatus for practicing the invention.

It has been discovered that the zone refining of bars or rods of metals and semi-conductor material may be carried out stisfactorily without the use of a supporting crucible or the like by passing through an elongated member being floating zone refined, an electrical current of such a magnitude that it produces an electromagnetic pinch effect at the molten zone and thereby maintains the molten material in place. This process can be applied not only to high resistance material such as silicon but it can be applied to metals in general.

More particularly, a bar of a metal, a semi-conductor Patented Jan. 14, 1%64 ice or other material, is provided with contacts or electrodes, at each end for instance, whereby electrical current can flow along a selected length of the bar. Electrical current at a suitable voltage is caused to flow longitudinally along this selected length of the bar. The magnitude of the electrical current is limited by that which is sufficient to heat the bar to an elevated temperature somewhat below its melting point. It will be understood that a substantial current on the order of from to 2,000 amperes for a bar of a radius of one centimeter may be required.

For semi-conductors or high resistance materials, it is desirable to preheat the rod or bar to a temperature just below its melting point, for example, 100 C. lower, before passing electrical current through the rod. The preheating reduces the resistivity to a reasonable value.

Thereafter, a relatively narrow zone within the length through which the electrical current is passing is heated by a suitable device in order to raise the temperature within that narrow zone above the melting point of the material; thereby a molten globule or zone of material is produced. The passage of the electrical current longitudinally along the length will produce an electromagnetic pinch eifect in the molten zone tending to constrict the molten globule. This pinch effect will add to and reinforce any surface tension or other forces that tend to maintain the molten globule in place. Consequently, a much thicker rod may be successfully zone refined, and much greater length of molten zone may be employed satisfactorily than would be possible without the passage of the electrical current along the length of the bar.

The passage of an electrical current longitudinally along the bar gives rise to an inwardly directed pressure on the exterior surface of the cylindrical molten zone. The magnitude of this pressure is directly proportional to the square of the current, I, and inversely proportional to the square of the radius, r. The pressure in dynes per square centimeter on the surface of the rod is indicated by the following equation:

(I is in amperes) As the radius of the bar increases, for the case where all the joule heating is dissipated by radiation and the current is limited thereby, the pinch effect becomes progressively more important in comparison to the surface tension efiect.

Referring to the figure of the drawing, there is shown an apparatus 16 for practicing the invention. The apparatus it comprises a base 12 provided with an insulator 14 through which passes an electrical conductor 16. The electrical conductor 16 is supported by a bushing 18 of procelain or the like above the base 12, the conductor 16 comprises a vertical portion 26 and an angle portion 22 which later terminates in a contact 24 having a hollow 26 into which the bar of material to be zone melted will fit in electrical contact. The base 12. is also provided with another insulator 28 through which passes a second electrical conductor 30 which is connected to a bottom contact member 32 provided with a cavity 34- into which the lower end of the bar to be processed will fit.

A bar 36 of the material to be zone refined has its upper end fitting within the upper hollow 26 while the lower end fits within the cavity 34. A source 38 of electrical current, which may be direct current or alternating current, may be connected to the conductors l6 and 36 with a switch 39 in the circuit enabling the circuit to be closed or open as desired.

A water cooled hollow induction coil 49 is disposed for relative movement through the base 2. The induction coil 40 is provided with a suitable rack 42 engageable with a gear 44 which coacts with the rack in order to move the induction coil up and down as desired. The induction coil 4% is connected to a source 45 or" high frequency current. The induction coil 40 comprises a suitable loop portion 48 disposed about the bar 36. it Will be appreciated that the induction loop 24 may comprise a pancake or other configuration depending on the requirements. It will be understood that the loop 48 may provide some electromagnetic levitation effect on the molten zone in order to further assist in maintaining the molten material in place. Disposed on the base 12 is a gas-tight ring 50 to which is fitted the bottom of a suitable enclosure 52 of glass or other material to en- .close in a hermetically sealed chamber the portions of the apparatus described herein which are disposed above the upper surface of the base 12. A suitable conduit 54 which may be connected to a source of vacuum or gas by suitable valving is provided in the base 12 in order to evacuate the chamber formed by the enclosure 52, or to admit a gas thereto.

It will be understood that the induction coil 4048 need not be employed, thus a graphite heat r ring capable of being energized by electrical current may be disposed closely about the bar 36 so as to radiate surficient heat to melt the desired zone of the bar or a reflecting ring to reflect enough heat radiated from the bar back to the molten zone.

Assuming that a bar of a metal of a resistivity of 100 microhm centimeters, and of a radius of one centimeter is disposed within the contacts 24 and 3-2, an electrical current, for example, of 1,000 amperes is passed through the conductors 16 and 39 and thus through the bar 36. A current of this magnitude will heat the metal to approximately 1000" C. assuming an emissivity of 0.5, and the pinch eflect will be 3080 dynes/cnr This temperature should be below the melting point of the metal. Titanium is an example of such metal. Thereafter, the induction coil. 45l-d8 is energized whereby high frequency currents will flow within and heat the portion of the metal immediately adjacent the loop 48. The temperature of the metal adjacent the loop Will rise above its melting point and a molten zone 54- will be produced. The molten zone is held in place by surface tension effects assisted by the electromagnetic pinch efiect of the electrical current flowing longitudinally through the bar 36 from contacts 24 to 34.

For a silicon bar of a radius of one centimeter the pinch effect, using a current of about 159 amperes which heats the rod to a temperature just below its melting point, assuming the emissivity of the molten silicon to be approximately 0.l, is approximately 7t) dynes per square centimeter. It the emissivity is greater, more current can be applied and the pinch pressure will be proportionately greater.

It will be understood that the electrodes 24 and 34 need not be applied to the ends of the bar being zone melted but may be applied at any selected portion along the length thereof within whichthe induction loop will be placed.

In practice the induction loop '43 will be placed at either the upper or lower end of the bar 36 and thus produce a molten zone 54. The induction loop Will be raised or lowered, as the case may be, to the other end of the bar -whereby zone refining of the silicon. is effected. Repetition of this relative movement of the molten zone along the length of the bar 36 will produce silicon that becomes purer with every passage. It will be understood, of course, that the bar 36 itself may be moved relative to the induction loop 48.

The passing of a molten zone along an aluminum rod 'Was carried out in accordance with the present invention. An aluminum rod of a length of 6 inches and of -ai idiameter,of one-quarter inch was clamped between lectrodes and a current of 240 amperes was passed longitudinally along the 6 inch length. There was a very substantial increase in the length of the molten zone that could be maintained while this 249 ampere current was being passed through the rod as compared to the length of molten zone that could be maintained without current. Thus, Without any current passing through the rod a molten zone of about two centirneters in length was the longest that could be obtained without the globule molten material suddenly breaking and running down. With the 240 amperes of current passing within the electrodes, a molten zone of approximately 3 centimeters could be readily maintained.

A six inch rod of copper of a diameter of one-quarter inch was similarly tested. The passage of an electrical current of several hundred amperes also resulted in approximately 50 percent increase in the length of the molten zone that could be supported as compared to the longest molten zone maintainable without passage of the electrical current.

It will be understood that the present invention enables the zone melting of rods of greater diameter than is possible by previously known procedures due to the passing of an electrical current longitudinally whereby the electromagnetic pinch effect operates to apply a constrictive pressure at the molten zone.

It is understood that the above-description is exemplary and not limited.

I claim as my invention:

1. In the process of floating Zone melting an unconfined vertically disposed bar of an electrically conductive material, the steps comprising passing an electrical current longitudinally along a selected length of the bar, the electrical current being in an amount of from to 2006 amperes per 1r square centimeters of cross-sectional area and being not more than that which will heat the length to an elevated temperature below its melting point, inductively heating a relatively narrow zone within said length to a temperature above the melting point of the material in said zone whereby a narrow molten zone is produced, the said electrical current passing through said molten zone causing an electromagnetic pinch effect which assistsin maintaining the molten material in place, and causing relative vertical movement of the molten zone by relative movement of the inductive heating along said length.

2. In the process of zone melting an unconfined bar of electrically conductive material, the steps comprising? passing an electrical current longitudinal along the bar, the current being in an amount of from 100 to 2,000 amperes per 7r square centimeters of cross-sectional area and being or a magnitude to heat a selected length of the bar to an elevated temperature below its melting point, heating a selected zone of the bar within said length to a temperature above the melting point of the material, the passage of the electrical current through the molten zone causing an electromagnetic pinch effect which assists in maintaining the molten zone in place, and causing rela: tive movement of the molten zone with respect to the bar;

3'. in the process .of zone refining an unconfined verti cally, disposed bar of silicon, the steps comprising passing an electrical current longitudinally along a length the bar of silicon, the magnitude of the electrical current being not in excess of about amperes per 11' square centimeters of cross-sectional area and being suficient to maintain the temperature of the length of the silicon bar at a point between above 500 C. and slightly below the melting point of the silicon, inductively heating a relatively narrow zone within said length to atemperature above the melting point of silicon whereby the zone comprises molten silicon, the said electrical current passing through the molten .zone causing an electromagnetic pinch effect which assists in maintaining the silicon in place and in contact with unmelted silicon in the por-- tions of the bar above and below the zone, and causing;

relative movement of the inductive heating along said length whereby relative movement of the molten Zone 1. occurs, the passage of said electrical current being maintained throughout the entire process.

4. In the process of zone refining an unconfined vertioally, disposed bar of silicon, the steps comprising preheating the bar of silicon to a temperature slightly below its melting point, passing an electrical current longitudinally along a length of the bar of silicon, the magnitude of the electrical current being not in excess of about 158 amperes per 1.- square centimeters of cross-sectional area and being sufiicient to maintain the temperature or the length of the silicon bar at a point between above 500 C. and slightly below the melting point of the silicon, inductively heating a relatively narrow zone within said length to a temperature above the melting point of silicon whereby the zone comprises molten silicon, the said electrical current passing through the molten zone causing an electromagnetic pinch effect which assists in maintaining the silicon in place and in contact with unmelted silicon in the portions of the bar above and below the zone, and causing relative movement or" the inductive heating along said length whereby relative movement of the molten zone occurs, the passage of said electrical current being mm'ntained throughout the entire process.

5. In the process of zone refining an unconfined vertically, disposed bar of semi-conductor material, the steps comprising preheating the bar of semi-conductor material to a temperature slightly below its melting point, passing an electrical current longitudinally along a length of the bar of semiconductor material, the magnitude of the electrical current being in an amount of from 100 to 2,000 amperes for 71' square centimeters of cross-sectional area and the current varying for other thicknesses of bar by the product of the radius in centimeters squared with this current and being sufiicient to maintain the temperature of the length of the semi-conductor material bar at a point between above 500 C. and slightly below the melting point of the semi-conductor material, inductively heating a relatively narrow zone Within said length to a temperature above the melting point of semiconductor material whereby the zone comprises molten semi-conductor material, the said electrical current passing through the molten zone causing an electromagnetic pinch eltect which assists in maintaining the semi-conductor material in place and in contact with unmeltecl semi-conductor material in the portions of the bar above and below the zone, and causing relative movement of the inductive heating along said length whereby relative movement of the molten zone occurs, the passage of said electrical current being maintained throughout the entire process.

References Cited in the file of this patent UNITED STATES PATENTS 2,743,199 Hull Apr. 24, 1956 2,789,039 Jensen Apr. 16, 1957 2,792,317 Davis May 14, 1957 OTHER REFERENCES Keck, Van Horn, 3. Soled, and A. MacDonald: The Review of Scientific instruments, vol. 25, No. 4, 331-334, April 1954.

Heywang: Z. Naturforsoh, 110, 238, 1956, pages 238- 243. 

1. IN THE PROCESS OF FLOATING ZONE MELTING AN UNCONFINED VERTICALLY DISPOSED BAR OF AN ELECTRICALLY CONDUCTIVE MATERIAL, THE STEPS COMPRISING PASSING AN ELECTRICAL CURRENT LONGITUDINALLY ALONG A SELECTED LENGTH OF THE BAR, THE ELECTRICAL CURRENT BEING IN AN AMOUNT OF FROM 100 TO 2000 AMPERES PER $ SQUARE CENTIMETERS OF CROSS-SECTIONAL AREA AND BEING NOT MORE THAN THAT WHICH WILL HEAT THE LENGTH TO AN ELEVATED TEMPERATURE BELOW ITS MELTING POINT, INDUCTIVELY HEATING A RELATIVELY NARROW ZONE WITHIN SAID LENGTH TO A TEMPERATURE ABOVE THE MELTING POINT OF THE MATERIAL IN SAID ZONE WHEREBY A NARROW MOLTEN ZONE IS PRODUCED, THE SAID ELECTRICAL CURRENT PASSING THROUGH SAID MOLTEN ZONE CAUSING AN ELECTROMAGNETIC PINCH EFFECT WHICH ASSISTS IN MAINTAINING THE MOLTEN MATERIAL IN PLACE, AND CAUSING RELATIVE VERTICAL MOVEMENT OF THE MOLTEN ZONE BY RELATIVE MOVEMENT OF THE INDUCTIVE HEATING ALONG SAID LENGTH. 